Tubular feedthrough system for hermetically sealed devices

ABSTRACT

An apparatus for hermetically sealing implantable devices, such as a microstimulator, moicrotransducer, microtelemeter, or the like, prevents entrapment of water vapors and other volatile gases, and allows hermeticity testing at all manufacturing levels. The venting of water vapors and other volatile gases is accomplished with a tubular feedthrough that allows such gases trapped within the sealed device to vent during the manufacturing process. The tubular feedthrough also establishes a conduit through which leakage tests or other hermeticity tests can be conducted prior to and after sealing the feedthrough. The tubular feedthrough, when made from conductive materials, also provides for electrical connections between electronic circuits sealed within the device and electrodes and other electrical terminals on the outside of the capsule.

This application is a Divisional of application Ser. No. 08/446,138,filed May 22, 1995, now issued as U.S. Pat. No. 5,640,764.

BACKGROUND OF THE INVENTION

The present invention relates to hermetically sealed cases or housingssuitable for implantation within living tissue, and more particularly,to a method and apparatus for hermetically sealing an implantable devicewhich prevents moisture from forming within the sealed device. In apreferred approach, the method and apparatus utilize a tubularfeedthrough to vent moisture and other volatile gases trapped within thesealed device during the manufacturing process. Such tubular feedthroughalso facilitates hermeticity testing of the implantable device duringthe manufacturing process.

Hermetically sealed cases or housings are widely used to protectelectronic or other components that may be susceptible to damage ormalfunction from exposure to the surrounding environment. The hermeticseal is simply an airtight, durable seal that is long-lasting andphysically rugged. Sometimes the interior of an hermetically sealedenclosure is filled with an inert gas such as helium, to further retardthe deterioration of the component or components inside. As no seal isperfect, the tightness of the hermetic seal, referred to as thehermeticity, is typically measured or specified in terms of the leakagerate through the seal, expressed in cc/sec of helium at standardtemperature and pressure. Sometimes, for very low leakage rates, thehermeticity can only be measured by placing a radioactive gas within theenclosure and then using an appropriate radiation detector to "sniff"the seal for radioactive leaks.

Where the electrical component or components are to be implanted in bodytissue, the hermetically sealed case (which must be made from a materialthat is compatible with body tissue, such as titanium, platinum orstainless steel or glass) serves a dual purpose: (1) it protects theelectrical or other components housed in the device from body fluids andtissue, which fluids and tissue could otherwise prevent the componentsfrom performing their desired function; and (2) it protects the bodytissue and fluids from the electrical or other components, whichcomponents may be made at least in part from materials that may bedamaging to body tissue, and which therefore could pose a potentialhealth risk to the patient wherein they are implanted. It is thuscritically important that the hermetic seal of an implanted device beespecially long-lasting and physically rugged. For this reason,stringent requirements are imposed on the hermeticity of an implanteddevice, typically requiring a seal that provides a helium leakage rateof less than 10⁻⁸ cc/sec at standard temperature and pressure (STP).

In recent years, the size of implanted medical devices has decreaseddramatically. It is now possible, for example, to construct a simplestimulator device in a small hermetically sealed glass tube that can beimplanted through the lumen of a needle. With such a small size comesincreased requirements for the tightness of the hermetic seal becausethere is less empty space inside of the sealed unit to hold the moisturethat eventually leaks therethrough. The hermeticity requirements of suchsmall devices may thus be on the order of 10⁻¹¹ or 10⁻¹² cc/sec. Whilethe small size is thus advantageous, the stringent hermeticityrequirements imposed for such small devices makes them extremelydifficult to manufacture, and thus increases the cost.

Most implanted medical devices, such as a cardiac pacemaker, neuralstimulator, biochemical sensor, and the like, require hermeticconductive feedthroughs in order to establish electrical contact betweenthe appropriate circuitry sealed in the hermetically closed case orcapsule and an external electrode that must be in contact with the bodytissue or fluids outside of the sealed case or capsule. In a pacemaker,for example, it is common to provide such a feedthrough by using afeedthrough capacitor. A representative feedthrough capacitor isdescribed in U.S. Pat. No. 4,152,540. Alternatively, a hermeticfeedthrough is typically used to establish electrical connectionsbetween the appropriate electronic components or circuitry sealed in thehermetically closed case or capsule and an external control device, ormonitoring equipment.

Heretofore, an hermetic feedthrough for implantable devices hasconsisted of a ceramic or glass bead that is bonded chemically at itsperimeter through brazing or the use of oxides, and/or mechanicallybonded through compression, to the walls of the sealed case or capsule.A suitable wire or other conductor passes through the center of thebead, which wire or conductor must also be sealed to the bead throughchemical bonds and/or mechanical compression. The feedthrough is thuscircular, and the wire(s) or conductor(s) mounted within the bead arecentered or mounted in a uniform pattern centrally positioned within thebead. Such centering is necessary due to the thermal coefficientsrequired for the different expansion rates that occur when heat isapplied to either create a compression seal or to create an oxide orbronze bond.

A significant problem associated with these hermetically sealed devices,particularly where the device is implanted in living tissue, is theinability to effectively seal the device with conventional feedthroughmechanisms. During existing sealing process, glass beads are fusedwithin the device cases or capsules to hermetically seal the case orcapsule. Water vapors are often produced by the gas flame fusing and areoften trapped inside the capsules which water vapors may lead toeventual failure of the implanted device.

Another problem associate with conventional sealing processes is relatedto the expansion of gases remaining inside the sealed capsule. As theglass bead is fused to the case or capsule, air inside the case orcapsule expands. The expanding air tries to escape from the case orcapsule and is likely to result in localized stress in the glass fusionareas and may even form a hole through the molten glass. If a hole formswithin the glass, it is very difficult if not impossible to repair.

Alternative methods of sealing microstimulator capsules involveexpensive equipment such as an infrared laser or a dedicated glass diodesealing machine which utilizes heated formed graphic holding blocks.

Other examples of the related art pertaining to hermetic feedthroughassemblies for implantable medical devices are disclosed in U.S. Pat.Nos. 4,678,868 issued to Kraska et al., and 4,940,858 issued to Tayloret al. While these patents are directed to feedthrough assemblies forimplanted medical devices, they do not address the problem of trappedwater vapors and expanding air within the sealed device.

Still other art relating generally to methods for forming hermeticallysealed cases having electrical feedthroughs and vias include U.S. Pat.No. 4,525,766 issued to Petersen, U.S. Pat. No. 4,861,641 issued toFoster et al., and U.S. Pat. No. 4,882,298 issued to Moeller et al.While these patents teach improvements in the art, such teachings arelimited to use with semiconductor substrates and are not easilyadaptable for use with microminiature devices implantable within livingtissue.

It is thus evident that what is needed is a cost-effective manner ofencapsulating implantable electronic or other devices that eliminates orprevents any moisture and expanding air from becoming trapped within thesealed device as the device is hermetically sealed. Further, once suchdevice is sealed, there is a need for a cost-effective, non-destructivemanner of testing the hermeticity of the sealed device during themanufacturing process.

SUMMARY OF THE INVENTION

The present invention addresses the above and other needs by providing alow cost method of encapsulating microstimulators and other implantabledevices through the use of a tubular feedthrough. The tubularfeedthrough allows moisture and expanding air trapped within the sealeddevice to vent during the manufacturing process. Further, the tubularfeedthrough facilitates hermeticity testing at all stages ofmanufacturing by providing a channel through which leakage testingapparatus may be attached to detect the presence of inert gases that areintroduced in the environment surrounding the encapsulated device andthat leak thereinto. Alternatively, an inert gas may be introduced intothe capsule (encapsulated device) under pressure using the tubularfeedthrough and thereafter the environment surrounding the capsule maybe tested to see if any of the inert gas has leaked out from thecapsule. Such leakage detection of the capsule can advantageously beaccomplished via the tubular feedthrough at any time prior to sealingthe feedthrough, or thereafter (in the case of inert gases beingintroduced into the capsule).

The present invention also permits electrical connection betweenelectronic circuits sealed within the case or capsule and electrodesand/or other electrical terminals on the outside of the case or capsule.That is, while preventing moisture and expanding air from becomingtrapped within the capsule as the assembly is sealed, as explainedabove, the tubular feedthrough may further provide a conductive paththrough the seal through which signal connections can be made.Alternative embodiments of the conductive tubular feedthroughcontemplate more than just electrical conduction, but also includeoptical conduction, fluid conduction, etc., to allow signal connectionsbetween the interior and exterior of the hermetically sealed case orcapsule.

BRIEF DESCRIPTION OF THE DRAWING

The above and other aspects, features and advantages of the presentinvention will be more apparent from the following more particulardescription thereof, presented in conjunction with the followingdrawings wherein:

FIG. 1 is a side, cross sectional view of a simple implantable devicethat is hermetically sealed in accordance with the present invention;and

FIG. 2 is a side, cross sectional view of a microstimulator having thetubular feedthrough hermetic seal of the present invention.

FIG. 3 is a flow diagram illustrating the method of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following description is the best mode presently contemplated forcarrying out the present invention. This description is not to be takenin a limiting sense, but is made merely for the purpose of describingthe general principles of the invention. The scope of the inventionshould be determined with reference to the claims.

The tubular feedthrough disclosed herein is adapted for use with manydifferent implantable electronic devices and particularly where it isadvantageous to prevent entrapment of water vapors and other volatilegases within the sealed device. The disclosed method and apparatus isalso adapted for use with hermetically sealed devices to allowhermeticity testing throughout the manufacturing process.

The preferred embodiment of the tubular feedthrough is particularlyadapted for use with microstimulators, microtransducers, microtelemetersor similar electronic devices, or combinations of such devices. Forexample, the invention may be used with the microstimulator of the typedisclosed in U.S. Pat. Nos. 5,193,539; and 5,193,540, incorporatedherein by reference. Such microstimulators typically comprises: acapsule which is generally tubular glass capillary having two open ends;an electronic assembly; one or more hermetic seals and feedthroughs; andone or more active electrodes. It is to be understood, however, that theinvention can be practiced with any type of implantable device wherein ahermetic seal is required, regardless of whether such device requireselectronic circuitry or not.

Referring now to FIG. 1, there is shown a general representation of asimple implantable device 13 that is hermetically sealed in accordancewith the methods described herein. The implantable device 13 includes ahermetically sealed casing 20, and a tube 30 that has one end 34disposed within the casing 20 and the other end 35 extending out fromthe casing 20. The tube 30 is hermetically sealed to the casing 20 withthe aid of a suitable seal 29 at the point of entry into the casing 20.In addition, an assembly 40, such as an electronic assembly, opticalassembly or other component or element may be disposed within the casing20 and connected to the tube 30 such that an appropriate connectionbetween the encased assembly 40 and the outside of the casing 30 isestablished. A seal 49 is disposed in the end 35 of the tube 30 afterany volatile gases within the casing 20 have been vented through thetube and after the implanted device 13 has been vacuum baked to driveout any moisture from the casing 20 through the tube 30. Such seal 49may comprise a simple plug, or simply a crimp in the tube 30, or a weld,or any other means for hermetically closing the end 35 of the tube 30.

Prior to sealing the end 35 of the tube 30, hermeticity testing of thecasing 20 can be easily accomplished in one of several manners. Forexample, by connecting a leakage detector 33 to the end 35 of the tube30 extending from the casing 20 and introducing an inert detectable gasin the environment outside the sealed casing 20, one may find defectiveseals in the casing 20 by detecting the presence of the inert gas thatleaks from the outside environment into the sealed casing 20.Alternatively, leaks may be detected by introducing an inert detectablegas into to the casing 20 via the tube 30 and subsequently detecting thepresence of any inert detectable gas using the leakage detector 33 thatleaks out of the casing 20.

Referring next to FIG. 2, a microstimulator 12 includes a first hermeticfeedthrough conductor 14 that is formed at the aft end 16 of a glasscapillary 20. The first feedthrough conductor 14 is preferably a metalwire 22 extending through a preformed glass bead 24. The preformed glassbead 24 is then placed within the aft end 16 of the glass capillary 20and is hermetically sealed therein by fusing the aft end 16 of the glasscapillary 20 with the glass bead 24 using a gas flame. A secondfeedthrough conductor 26 is formed by sealing a second glass bead 28around a metal tubing 30. This second glass bead 28 is also dimensionedto snugly fit into the open forward end 32 of the glass capillary 20.The aft end 34 of the metal tubing 30 is then connected to themicrostimulator electronic assembly 40 proximate the forward end 36thereof.

The microstimulator electronic assembly 40 is then slid into the glasscapillary 20. The radial dimension of the glass capillary 20 is suchthat the microstimulator electronic assembly 40 can be slid thereinleaving a small clearance therebetween. The longitudinal dimensions ofthe microstimulator electronic assembly 40 and the glass capillary 20are such that when the aft end 38 of the microstimulator electronicassembly 40 reaches the first conductive feedthrough 14, the openforward end 32 of the glass capillary 20 aligns with the second glassbead 28. A conductive epoxy connection 44 is made between the aft end 38of the microstimulator electronic assembly 40 and the first hermeticfeedthrough conductor 14.

Hermetic sealing of the forward end 32 of the glass capillary 20 isaccomplished by fusing the forward end 32 of the glass capillary 20 withthe second glass bead 28 using a gas flame. The metal tube 30 extendingfrom the microstimulator electronic assembly 40 through the second glassbead 28 allows the expanded air and moisture to escape during the fusingprocess. After the glass bead to capillary sealing, the entiremicrostimulator 12 is vacuum baked until all the moisture and othervolatile contaminates are driven out of the microstimulator capsule. Ifdesired an inert gas, such as helium or argon, may be pumped into thecapsule. The forward end 46 of the metal tube 30 is then sealed orpinched off with a very small flame or some other sealing method such asa tungsten inert gas (TIG) flame, resistance welding, or laser welding.A microstimulator electrode wire 50 can be attached to the metal tube30, as needed, during the pinching process or in a subsequentlyperformed attachment step.

The method of forming a tubular feedthrough for an implanted electronicdevice in accordance with the present invention comprises threeessential steps including: (a) attaching one end of a metal tube to anelectronic assembly such that an electrical connection or feedthrough isestablished; (b) encasing the electronic assembly within a capsule suchthat one end of the metal tube resides in the interior of the sealedcapsule while the other end extends out from of the capsule; and (c)hermetically sealing the metal tube to the capsule. The steps areperformed in such a manner that the tubular feedthrough vents anymoisture and other volatile gases that are trapped within the capsuleand further allows hermeticity testing or leak detection of the sealedcapsule. Changes in the order of the aforementioned steps may be madewithout sacrificing the advantages presented by this method.

A low cost method of hermetically sealing microstimulators,microtransducers, microtelemeters, and other implantable electronicdevices such that moisture and expanding air trapped within the deviceare effectively vented, is illustrated in the flow diagram of FIG. 3.The method essentially comprises four steps. The first step (block 60 ofFIG. 3) involves forming a venting feedthrough by having a glass beadhermetically sealed around a platinum-iridium tube. The platinum-iridiumtube is preferably a small diameter tube on the order of about 0.25 mmoutside diameter by 0.125 mm inside diameter by about 2.5 mm in length.One end of the platinum iridium tubing is attached to the forward end ofthe microstimulator electronic assembly forming an electrical connectiontherewith.

The second step involves fusing the glass bead to a suitable glasscapsule such that the microstimulator electronic assembly is fullyencased within the capsule (block 62 of FIG. 3). The glass bead andglass capsule are preferably dimensioned such that the glass bead snuglyfits into the open end of the glass capsule leaving little or noclearance therebetween. The glass-to-glass fusing is preferably doneusing a gas-oxygen flame (or, as indicated below, an infrared laser) andis performed at a temperature of about 1180°-1200° C. (e.g., 1189° C.)being careful not to sustain heat damage to the microstimulatorelectronic assembly. During the glass fusing process, water vapors areproduced by the gas-oxygen flame which heretofore would have beentrapped inside the glass capsule. In addition, any air inside thecapsule will expand due to the increased temperature. Advantageously,the platinum-iridium tube extending from the microstimulator electronicassembly through the glass bead, provides a path through which theexpanded air and moisture are vented.

Alternatively, in lieu of using a gas-oxygen flame, an infrared lasercould be used to seal the end of the tube. An infrared laser isparticularly well suited for this operation because it may be used tomelt the glass in an inert atmosphere, such as argon gas, and as aresult no water is generated as the melting operation is performed.Hence, unlike the gas-oxygen flame, which produces water as one of itsproducts, and which water could easily end up inside of the hermeticseal, the infrared laser (when used in an inert atmosphere) prevents theformation of water.

The third step in the described method for hermetically sealingmicrostimulators is vacuum baking of the entire microstimulator in anevacuated oven until all the moisture and other unwanted gases aredriven out of the microstimulator capsule (block 64 of FIG. 3). Thevacuum baking step is preferably performed at a temperature of about 80°C., for approximately 72 hours, again being careful not to damage themicrostimulator electronic assembly.

Next, the microstimulator is cooled (block 66), and then themicrostimulator electrode wires, or other wires needed for operation ofthe device, may then be inserted into the open end of theplatinum-iridium tubing (block 68 of FIG. 3). The microstimulatorelectrode wires are also made from platinum-iridium and are dimension tofit within the platinum-iridium tube. The platinum-iridium electrodewire and platinum-iridium tube are then hermetically sealed (block 70 ofFIG. 3) with a process such as resistance welding, a TIG flame, or laserwelding. It should also be noted that the tube itself may be sealedwithout inserting an electrode wire therein, and the tube then functions(when properly electrically connected to the appropriate circuitrywithin the microstimulator) as the electrode wire. It is further notedthat other dissimilar metals, which do not corrode, and which have anappropriate corresponding coefficient of expansion, may be used in lieuof platinum-iridium as the tube material.

Optionally, inert detectable gases, such as helium, may be introducedinto the microstimulator capsule just prior to the final hermeticsealing. This additional step facilitates hermeticity testing at allsubsequent stages of manufacturing. By detecting the presence of theseinert gases outside the sealed capsule using various leakage tests,defective devices can be identified and receive the appropriatedisposition.

From the foregoing, it should be appreciated that the present inventionthus provides an improved method and apparatus for hermetically sealingmicrostimulators and other implantable electronic devices that preventsmoisture and expanding air from becoming trapped within the sealeddevice. Further, it will be apparent that various changes and additionsmay be made in the described methods and in the form, construction andarrangement of the elements thereof without departing from the spiritand scope of the invention or sacrificing all of its materialadvantages, the forms and methods hereinbefore described being merelyexemplary embodiments thereof. Therefore, it is not intended that thescope of the invention be limited to the specific embodiments andprocesses described. Rather, it is intended that the scope of thisinvention be determined by the appending claims and their equivalents.

What is claimed is:
 1. A tubular feedthrough system for an implanteddevice comprising:a casing, a tube having one end positioned within thecasing and the other end extending out of the casing, the tube beinghermetically sealed to the casing so that the only non-sealed openinginto the casing is through the tube, the non-sealed tube therebyfunctioning as a means for venting moisture and other volatile gasesformed within the casing, and sealing means for sealing the end of thetube extending out of the casing after the moisture and other volatilegases have been vented therethrough, thereby hermetically sealing thecasing.
 2. The tubular feedthrough system of claim 1 further comprisinga means for detecting leaks in the casing via the tube prior to sealingthe tube.
 3. The tubular feedthrough system of claim 2 wherein the meansfor detecting leaks in the casing further comprises a means forinjecting an inert gas through the tube prior to sealing the tube withthe sealing means.
 4. The tubular feedthrough system of claim 1 whereinthe tube is a conductive tube.
 5. The tubular feedthrough system ofclaim 4 further comprising an electronic assembly disposed within thecasing and connected to the tube such that an electrical connection isestablished from the outside of the casing to the electronic assemblyvia the conductive tube so that the tube may pass electrical signalsbetween a location inside of the casing to a location outside of thecasing, whereby the tube functions as an electrode for the electronicassembly.
 6. The tubular feedthrough system of claim 5 wherein theimplanted device comprises a microstimulator.
 7. The tubular feedthroughsystem of claim 6 wherein the sealing means for sealing the tubecomprises a wire inserted into the end of the tube and means forhermetically sealing the wire therein.
 8. The tubular feedthrough systemof claim 5 wherein the implanted device comprises a microtransducer. 9.The tubular feedthrough system of claim 5 wherein the implanted devicecomprises a microtelemeter.
 10. A tubular feedthrough system for animplantable device comprising:a casing (20) adapted for implantation inliving tissue, said casing having an opening (32) therein; a tube (30)having a first end, a second end, and a passageway therethroughconnecting the first end with the second end; means for hermeticallysealing a sealing element (28, 29) around the tube; means forhermetically sealing the sealing element to the opening in the casingsuch that the first end of the tube resides inside of the casing whilethe second end of the tube resides outside of the casing, the passagewaythrough the tube thereby providing a vent hole through which volatilegases within the casing may escape; means for hermetically sealing thetube.
 11. The tubular feedthrough system of claim 10 further comprisingan assembly (40) attached to the first end of the tube so as to bedisposed within the casing when the sealing element (28, 29) is sealedto the casing.
 12. The tubular feedthrough system of claim 11 whereinthe assembly within the casing comprises an electronic assembly.
 13. Thetubular feedthrough system of claim 11 wherein the assembly within thecasing comprises an optical assembly.
 14. The tubular feedthrough systemof claim 11 wherein the tube comprises a metal tube that functions as anelectrical feedthrough for making electrical contact with the assembly.15. The tubular feedthrough system of claim 11 wherein the means forhermetically sealing the tube comprises an electrode wire (50).
 16. Thetubular feedthrough system of claim 11 wherein the casing comprises aglass capillary.